Many homes and other buildings rely upon furnaces to provide heat during cool and/or cold weather. Typically, a furnace employs a burner that burns a fuel such as natural gas, propane, oil or the like, and provides heated combustion gases to the interior of a heat exchanger. The combustion gases typically proceed through the heat exchanger, are collected by a collector box, and then are exhausted outside of the building via a vent or the like. In some cases, a combustion blower is provided to pull combustion air into the burner, pull the combustion gases through the heat exchanger into the collector box, and to push the combustion gases out the vent. At the same time, a circulating air blower typically forces return air from the building, and in some cases ventilation air from outside of the building, over or through the heat exchanger, thereby heating the air. The heated air is subsequently routed throughout the building via a duct system. A return duct system is typically employed to return air from the building to the furnace to be re-heated and then re-circulated.
In order to provide improved fuel efficiency and/or occupant comfort, some furnaces may be considered as having two or more stages, i.e., they can operate at two or more different burner firing rates, depending on how much heat is needed within the building. Some furnaces are known as modulating furnaces, because they can potentially operate at a number of different firing rates and/or across a range of firing rates. The firing rate of the furnace typically dictates the amount of gas and combustion air that is required by the burner. The amount of gas delivered to the burner is typically controlled by a variable gas valve, and the amount to combustion air is often controlled by a combustion blower. For efficient operation, the gas valve and the combustion blower speed need to operate in concert with one another, and in accordance with the desired firing rate of the furnace.
In some cases, the variable gas valve is a pneumatic amplified gas/air valve that is pneumatically controlled by pressure signals created by the operation of the combustion blower. As such, and in these cases, the combustion blower speed may be directly proportional to the firing rate. Therefore, an accurate combustion blower speed is required for an accurate firing rate. When the furnace is first installed, and/or during subsequent maintenance, a calibration process must often be performed by the installer to correlate the combustion blower speed with firing rate, which in some cases, can be a relatively time consuming and tedious process.